In communications antenna applications, there is sometimes a need to generate an antenna beam which can be scanned electronically to cover a very wide range of pointing angles, the required coverage spanning each of two dimensions. An example is the antenna which forms part of an aircraft electronic system which communicates back to earth via a satellite. The beam of the aircraft antenna must be pointed in the direction of the satellite. However, according to the latitude and longitude location of the aircraft, the line of sight to the satellite may lie at low elevation, that is, very near to the horizon. Alternatively, it may lie directly overhead of the aircraft. Yet again, it may lie at some point between these two extremes. In addition, relative to the aircraft, the azimuth bearing of the satellite may be any angle within a 360.degree. range, according to the position of the satellite, and also according to the heading of the path along which the aircraft is flying. Put simply, the aircraft antenna needs to provide for an angular coverage which is specified by an area above the aircraft which covers almost a hemisphere.
In general, the angular position of an antenna beam may be changed either by physically tilting the antenna structure, or by scanning the beam electronically. In the latter case, the antenna structure does not move, and the antenna is designated as being a phase array. This latter device is configured as a number of discrete, small antenna elements. Ordinarily, each of these elements 1 is connected to an electronic phase-shifter device 3 which are coupled to a power splitter 5 to which an input signal is applied, as shown in FIG. 1. There exists a variant which is useful in some particular circumstances, in which several discrete elements are connected to another, forming a subarray. In this variant, a single electronic phase-shifter is used to excite each subarray. The phased array approach is preferred for use on an aircraft because a stationary antenna can have a low profile with minimal protrusions on the outside of the aircraft, and the design has a high reliability since there are no moving parts.
For electronic beam scanning, one requirement is that the design must incorporate appropriate electronic phase shifters, and their electronic control circuits. However, for large angular coverage, and in particular for the hemisphere aircraft communication application, there is another important requirement. This requirement is that each of the discrete radiating elements which is used in the design should, when excited as an antenna in its own right, generate a very wide antenna beam which has an almost constant pattern level over the scan range that is ultimately intended for the array. Thus, to meet the aircraft communication requirement, a single element should radiate at fairly constant level at any point on a hemisphere.
The requirement for the single element may be restated in more precise terms. Given that one principal end application involves a phased array, what is important is the immersed element pattern of the element. The immersed element pattern is the pattern which results when a single element is excited by a signal source, but the remaining elements of the array are nevertheless present in a physical sense. When predicting or measuring the immersed element pattern, none of these surrounding elements are connected to the signal source at any particular instant in time, but each of their terminals do connect to an individual matched load. It is the immersed element pattern as defined in this way which should have the desired broad-beam characteristic over almost a hemisphere.
The requirements for a single element of the phased array antenna which communicates with a satellite that uses circular polarization are that
(a) The element should be of a circularly polarized type. PA1 (b) The element should be of a very low profile in order that the aerodynamic properties of the aircraft fuselage are not unfavourably impacted (in practice, the phased array elements are accommodated under a radome which is mounted on the outside of the aircraft). The height of this radome is dictated essentially by the height of the elements. For example, it is preferred that an element height should not exceed 8 cm. at the Inmarsat L-band communication frequencies (1525 to 1660 MHZ). PA1 (c) The element should provide "near-hemisphere" coverage characteristics, as discussed above.
It is quite difficult to satisfy all of the above requirements. Of the antenna types that have been described in the published literature, two appeared initially to be promising: a configuration based on drooping dipoles, and the quadrifilar helix as described in C. C. Kilgus, "Resonant Quadrifilar Helix", IEEE Trans. AP-17, May 1969, pp 349-351, and in S. Foo, "A Quadrifilar Helical Antenna For Low Elevation GPS Applications", Microwave Journal, January 1998, pp. 179-184. The quadrifilar helix generates circularly polarized signals, and can be designed so as to give the required wide beam coverage. However its height (about 15 cm for a typical design at L-band) is too great in the aircraft application. Furthermore, for the quadrifilar helix design, the excitation signals would be applied via four coaxial cables which run up the center axis of the element. It would be awkward to provide an interface for this type of feed arrangement, given that the feed circuitry and electronic phase shifters should preferably be incorporated in a strip line device lying just below the elements.
A class of resonant quadrifilar helix antenna called a volute is described in the text "Antenna Engineering Handbook" (second edition), by Richard C. Johnson and Henry Jasik, pp. 13-19 to 13-20. The antenna consist of two orthogonal fractional-turn bifilar helices excited in phase quadrature. This type of antenna is capable of radiating a signal with circular polarization in a cardiod shape pattern. The antenna can be used to provide the wide beamwidth required in the above-described phased array application, over a relatively narrow frequency range.
As noted in the above text, the half-turn, half-wavelength volute is of particular interest because the input impedance of each bifilar can be matched to a 50 ohm coaxial input by minor adjustments of the helical arm lengths without the need of a transformer.
However, such antennae have been very expensive to manufacture. For microwave frequencies, and in particular for airborne or spaceborne applications, the half-wave volute elements are too large, especially when a large number are to be disposed in a phased array.
A solution to the cost problem was described in U.S. Pat. No. 4,686,536 issued Aug. 11, 1987, by David Allcock, and assigned to Canadian Marconi Company. This patent describes conventional drooping crossed dipole elements as described in the aforenoted text, and as on pp. 28-7 ff. thereof. The radiating elements are disposed on printed circuit boards as described in "Radscan A Novel Conically Scanning Tracking Feed", by Arthur Sullivan, Electro Magnetic Processes, Inc, pp. 247-256. However, the radiating elements and the feeds are disposed as microstrip elements on orthogonally arranged printed circuit boards. Unfortunately, the process of interfacing the two planar microstrip circuit boards and tuning the sensitive feed line between the boards is complex, and therefore is costly in production. As shown in FIG. 2, a balun 7 above a ground plane 8 is used to match the dipole elements 9 to a single feeder line 11 which is also spaced from the ground plane 8.
We have discovered that the low angle axial ratio and gain of the antenna described in the above-noted patent is not optimal. It is also clear that the feeder lines to the dipole elements are not balanced.